The present invention relates generally to surface cleaning and, more particularly, to a method and apparatus for applying a hybrid cleaning scheme to opposing sides of a substrate.
Megasonic cleaning is widely used in semiconductor manufacturing operations and can be employed in a batch cleaning process or a single wafer cleaning process. For a batch cleaning process, the vibrations of a megasonic transducer creates sonic pressure waves in the liquid of the cleaning tank which contains a batch of semiconductor substrates. A single wafer megasonic cleaning process uses a transducer of a size generally less than the area of the wafer substrate above a rotating wafer, wherein the transducer is scanned across the wafer, or in the case of full immersion a single wafer tank system is used. In each case, the main particle removal mechanisms with megasonic cleaning are due to cavitation and acoustic streaming. Cavitation is the rapid forming and collapsing of microscopic bubbles in a liquid medium under the action of sonic agitation. Upon collapse, the bubbles release energy which assists in particle removal through breaking the various adhesion forces which adhere the particle to the substrate. Acoustic streaming is the fluid motion induced by the acoustic wave transmission through the fluid.
FIG. 1A is a schematic diagram of a batch megasonic cleaning system. Tank 100 is filled with a cleaning solution. Wafer holder 102 includes a batch of wafers to be cleaned. Transducer 104 creates pressure waves through sonic energy with frequencies near 1 Megahertz (MHz). These pressure waves, in concert with the appropriate chemistry to control and inhibit particle re-adhesion, provide the cleaning action. Because of the long cleaning time required for batch cleaning systems, as well as chemical usage, efforts have been focused on single wafer cleaning systems in order to decrease chemical usage, increase wafer-to-wafer control, and decrease defects in accordance with the International Technology Roadmap for Semiconductors (ITRS) requirements. Batch systems suffer from another disadvantage in that the delivery of megasonic energy to the multiple wafers in the tank is non-uniform and can result in ‘hot spots’ due to constructive interference, or ‘cold spots’ due to destructive interference, each being caused by reflection of the megasonic waves from both the multiple wafers and from the megasonic tank. Therefore, a higher megasonic energy must be applied in order to reach all regions of the wafers in wafer holder 102.
FIG. 1B is a schematic diagram of a single wafer cleaning tank. Here, tank 106 is filled with a cleaning solution. Wafer 110, supported by carrier 108, is submerged in the cleaning solution of tank 106. Transducer 104 supplies the energy to clean wafer 110. The cleaning solutions are typically designed to modify the zeta potential between the surfaces of the wafer and a particle removed from the surface of the wafer through the acoustic energy supplied by transducer 104. The cleaning solution concentration should be maintained within a fairly tight range in order to maintain a suitable zeta potential between the surfaces. However, for features such as lines, contacts, spaces, vias, etc., defined on a surface of the substrate, the particle may redeposit on the surface of the substrate due to the inability to maintain a specific cleaning solution concentration, i.e., replenish the cleaning solution, within the region defined by the feature. Another shortcoming of the single wafer cleaning tank configuration is that the side of the substrate not facing the megasonic transducer does not see the megasonic energy. Since upwards of 90% of the megasonic energy is absorbed by the substrate, the opposing side is effectively shielded form the megasonic energy. As a result, in order to clean the opposing side, the substrate must be flipped or transferred to an additional cleaning station. Consequently, the throughput is reduced because of the additional processing.
In view of the foregoing, there is a need for a method and apparatus to provide a single wafer megasonic cleaning configuration that is capable of cleaning both sides of a substrate effectively and simultaneously.